Abstract
SummaryBackgroundLateral flow devices (LFDs) for rapid antigen testing are set to become a cornerstone of SARS-CoV-2 mass community testing, although their reduced sensitivity compared with PCR has raised questions of how well they identify infectious cases. Understanding their capabilities and limitations is, therefore, essential for successful implementation. We evaluated six commercial LFDs and assessed their correlation with infectious virus culture and PCR cycle threshold (Ct) values.MethodsIn a single-centre, laboratory evaluation study, we did a head-to-head comparison of six LFDs commercially available in the UK: Innova Rapid SARS-CoV-2 Antigen Test, Spring Healthcare SARS-CoV-2 Antigen Rapid Test Cassette, E25Bio Rapid Diagnostic Test, Encode SARS-CoV-2 Antigen Rapid Test Device, SureScreen COVID-19 Rapid Antigen Test Cassette, and SureScreen COVID-19 Rapid Fluorescence Antigen Test. We estimated the specificities and sensitivities of the LFDs using stored naso-oropharyngeal swabs collected at St Thomas' Hospital (London, UK) for routine diagnostic SARS-CoV-2 testing by real-time RT-PCR (RT-rtPCR). Swabs were from inpatients and outpatients from all departments of St Thomas' Hospital, and from health-care staff (all departments) and their household contacts. SARS-CoV-2-negative swabs from the same population (confirmed by RT-rtPCR) were used for comparative specificity determinations. All samples were collected between March 23 and Oct 27, 2020. We determined the limit of detection (LOD) for each test using viral plaque-forming units (PFUs) and viral RNA copy numbers of laboratory-grown SARS-CoV-2. Additionally, LFDs were selected to assess the correlation of antigen test result with RT-rtPCR Ct values and positive viral culture in Vero E6 cells. This analysis included longitudinal swabs from five infected inpatients with varying disease severities. Furthermore, the sensitivities of available LFDs were assessed in swabs (n=23; collected from Dec 4, 2020, to Jan 12, 2021) confirmed to be positive (RT-rtPCR and whole-genome sequencing) for the B.1.1.7 variant, which was the dominant genotype in the UK at the time of study completion.FindingsAll LFDs showed high specificity (≥98·0%), except for the E25Bio test (86·0% [95% CI 77·9–99·9]), and most tests reliably detected 50 PFU/test (equivalent SARS-CoV-2 N gene Ct value of 23·7, or RNA copy number of 3 × 106/mL). Sensitivities of the LFDs on clinical samples ranged from 65·0% (55·2–73·6) to 89·0% (81·4–93·8). These sensitivities increased to greater than 90% for samples with Ct values of lower than 25 for all tests except the SureScreen fluorescence (SureScreen-F) test. Positive virus culture was identified in 57 (40·4%) of 141 samples; 54 (94·7%) of the positive cultures were from swabs with Ct values lower than 25. Among the three LFDs selected for detailed comparisons (the tests with highest sensitivity [Innova], highest specificity [Encode], and alternative technology [SureScreen-F]), sensitivity of the LFDs increased to at least 94·7% when only including samples with detected viral growth. Longitudinal studies of RT-rtPCR-positive samples (tested with Innova, Encode, and both SureScreen-F and the SureScreen visual [SureScreen-V] test) showed that most of the tests identified all infectious samples as positive. Test performance (assessed for Innova and SureScreen-V) was not affected when reassessed on swabs positive for the UK variant B.1.1.7.InterpretationIn this comprehensive comparison of antigen LFDs and virus infectivity, we found a clear relationship between Ct values, quantitative culture of infectious virus, and antigen LFD positivity in clinical samples. Our data support regular testing of target groups with LFDs to supplement the current PCR testing capacity, which would help to rapidly identify infected individuals in situations in which they would otherwise go undetected.FundingKing's Together Rapid COVID-19, Medical Research Council, Wellcome Trust, Huo Family Foundation, UK Department of Health, National Institute for Health Research Comprehensive Biomedical Research Centre.
Highlights
Interpretation In this comprehensive comparison of antigen lateral flow devices (LFDs) and virus infectivity, we found a clear relationship between cycle threshold (Ct) values, quantitative culture of infectious virus, and antigen LFD positivity in clinical samples
Among three LFDs selected for detailed comparisons, we found a sensitivity of at least 94·7% when compared with samples that were infectious in vitro, with absolute viral titre in the specimens correlating with Ct values
COVID-19 continues to have a profound impact on global health, with many countries resorting to economically and socially damaging restrictions to minimise the spread of SARS-CoV-2 and protect health-care systems from being overwhelmed
Summary
COVID-19 continues to have a profound impact on global health, with many countries resorting to economically and socially damaging restrictions to minimise the spread of SARS-CoV-2 and protect health-care systems from being overwhelmed. In addition to the existing PCR-based testing systems, mass community testing might take the form of targeted intensive testing in areas of increasing incidence, alongside regular routine screening in health-care, education, workplace, and leisure settings. There are concerns about the reduced sensitivity of LFDs in comparison with PCR, and controversies have emerged over the suitability of their implementation.[3,4,5] Problems with comparing cycle threshold (Ct) values from RT-PCR between different protocols, and even between the same protocols at different locations, combined with uncertainty about the range of viral loads that constitute a transmission risk, have been the cause of many of the controversies.[5,6] Individuals are most infectious around the time of symptom onset, when viral loads in the upper respiratory tract are highest,[7,8] with recent studies confirming an association between viral load and increased transmission of SARS-CoV-2.9 For asymptomatic individuals, infectivity and viral load dynamics involve a similar, limited period of infectivity. Asymptomatic and presymptomatic contributions to viral spread in the community remain problematic, accounting for a notable proportion of transmissions but often going undetected.[10,11]
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